Looking at the roll period from 90 degrees to 270 degrees ( Zero being upright) with the paddler hanging stiff does not strike me very relevant to how easy the boat is to roll. The interesting part of rolling is going from 180 to 360 degrees. It is this portion of the rotation that is important, and the hardest part of the roll is typically from 270 to 360 degrees so determining the damping of the inverted boat (90 to 270) does not even address the section of the roll where most people (who know how to roll) fail. I was attempting to teach a friend to roll yesterday, he could always get his body to the surface of the water and the boat rotated to 90/270 degrees. He just couldn't rotate it beyond towards vertical. If the paddler is hanging stiff he would provide resistance. But as Bruce said, typically the paddler moves his body counter to the rotation of the boat, so instead of providing resistance to the roll, his body provides assistance, or at least very little resistance. The body drag is more of a factor during the setup, but this is not a factor of boat design. While inverted drag may contribute to how easily a boat rolls, it seems to me that the hardest part of the roll does not include the inverted section. Why evaluate the whole chain when you can just look at the weakest link. I think the most useful evaluation of rollability should concentrate on the part of the roll which is hard. If this is the case, the rotational drag that would seem to matter the most would be from 270 to 360 (or 90 to 0). This would be a good thing because trying to estimate the drag of the paddler is messy where modeling the drag of things such as rudders, skegs and hard chines should be a little more straight forward. > > >> I question whether the paddlers body adds much damping which is >significant >> to how easily the boat rolls. (SNIP) > >If the water flows across the body the body has an effect on damping. How >much effect? If you use a drag factor of 1.0 (probably reasonable for the >body but I would gladly accept something else if you have data) then you >need only calculate the rotational speed to determine how much resistance >the body offers. Assume an area of 2 square feet and given 1 second to >rotate 90 degrees it looks like 1.57' p/sec. Using Joessel's formula that >gives roughly 4.33 pounds of resistance, about the same resistance of most >boats traveling ahead at four knots. Now I would not state this as gospel >but it seems more than insignificant even allowing for some error. > >> I agree that damping will play a part, however again since the hardest >part >> of the roll seems to be the last 100 degrees or so, I don't think that >> upspwept ends will play a large part. A rudder would tend to be more of a >> hinderance than an upswept bow. > >I suppose the importance depends upon the center of rotation. In any case we >have to look at the sum of all damping effects not just one. maybe the >rudder, body, bow, section shape all add up to a significant whole. > > >> I don't see that the rolling period of the inverted boat would be that >> pertinant as it would mainly provide information for a orientation which >> most people don't have any problem getting beyond. > >I may have misread you on that. I got the impression you asked about the >effect of boat shape on rolling not the just the effect on rolling when >rolling got difficult alone. > > >> Timing how long it takes the boat to go from it's capsize point to fully >> inverted may provide more complete information. But even this would put >too >> much emphasis on the inverted characteristics. > >Why? > >> It seems to me that a boat that is easy to roll will be easy to roll >showly >> or quickly. Damping would be related to how fast the boat rotates. This >> would suggest that the dynamic part of the equation would be less >important >> to whether a boat is easy to roll. > >I am not sure I understand your point. > >Cheers, > >John Winters >Redwing Designs >Web site address http://home.ican.net/~735769 > >*************************************************************************** >PaddleWise Paddling Mailing List - All postings copyright the author and not >to be reproduced outside PaddleWise without author's permission >Submissions: PaddleWise_at_PaddleWise.net >Subscriptions: PaddleWise-request_at_PaddleWise.net >Website: http://www.paddlewise.net/ >*************************************************************************** Nick Schade Guillemot Kayaks 824 Thompson St, Suite I Glastonbury, CT 06033 (860) 659-8847 Schade_at_guillemot-kayaks.com http://www.guillemot-kayaks.com/ >>>>"It's not just Art, It's a Craft!"<<<< *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************Received on Wed Jul 05 2000 - 08:34:20 PDT
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